1. Field of the Invention
The invention relates to a rotation detection apparatus, a rotation angle detection apparatus, and an electric power steering system.
2. Description of the Related Art
Electric power steering systems (hereinafter referred to as EPSs) have been known which generate an assist torque using a motor as described in, for example, Japanese Patent Application Publication No. 2011-095094 (JP 2011-095094 A). A control apparatus in the EPS controls a torque to be generated by a motor in accordance with a steering torque detected through a torque sensor. The control apparatus conducts a current through a coil of the motor in accordance with a rotation angle of the motor detected through a rotation angle sensor.
EPSs are also available which perform compensation control on a basic target assist torque based on the steering torque in order to provide more excellent steering feeling. The compensation control includes steering return control that returns a steering wheel to a neutral position based on a steering angle of the steering wheel. The control apparatus in the EPS determines the steering angle by calculating a rotation angle of the steering shaft, for example, based on the rotation angle of the motor.
As described in JP 2011-095094 A, rotation angle sensors are known which include a magnetic sensor. Some such magnetic sensors each have two detectors. A first detector generates a sine signal that varies like a sine wave according to the rotation angle of a rotating shaft of the motor. A second detector generates a cosine signal that varies like a cosine wave according to the rotation angle of the rotating shaft of the motor. The control apparatus in the EPS calculates arctangent values for the sine signal and the cosine signal to determine the rotation angle of the rotating shaft.
In this case, the rotation angle detected through the rotation angle sensor is a relative angle varying within the range from 0° to 360°. In contrast, the steering angle, used for steering return control and the like, is an absolute angle varying beyond the range from 0° to 360°. Thus, with the rotation angle sensor in use, the steering angle needs to be detected in absolute value. The control apparatus in the EPS, for example, counts the number of rotations corresponding to the number of changes, during one period (360°), in the rotation angle (electrical angle) detected by the rotation angle sensor, and calculates the steering angle in absolute value based on the counted number of rotations and the rotation angle detected through the rotation angle sensor.
EPSs are available which power down the control apparatus and the rotation angle sensor when a power switch of a vehicle is turned off. In such an EPS, the rotation angle of the motor cannot be detected while the power switch is off. In this situation, when the steering shaft is rotated through operation of the steering wheel, the count value of the number of rotations is different from the actual value of the number of rotations. In this case, when the power switch of the vehicle is turned on again, the absolute value of the steering angle may fail to be appropriately detected.
To eliminate such concern, for example, Japanese Patent Application Publication No. 2014-234072 (JP 2014-234072A) adopts the following configuration. That is, the control apparatus in the EPS is provided with a control apparatus that controls driving of the motor and an arithmetic, circuit that calculates the rotation angle of the motor; the control apparatus and the arithmetic circuit are independent of each other. When the power switch on the vehicle is turned off, power from an in-vehicle battery or a battery provided separately from the in-vehicle battery is supplied to the rotation angle sensor and to the arithmetic circuit. The arithmetic circuit continues to calculate the rotation angle of the motor even while the power switch is off.
The EPS needs to have high reliability, and this also applies to the arithmetic circuit, which is a component of the EPS. For example, if the arithmetic circuit incorrectly calculates the rotation angle of the motor, driving of the motor is controlled based on the incorrect rotation angle. Thus, generating an appropriate assist torque may be difficult. Therefore, an abnormality in the arithmetic circuit needs to be appropriately detected.